Approximately 50% of cancer patients receive radiation therapy for treatment of their disease. A majority of the treatments are divided into fractions, in which radiation doses are delivered daily over several weeks. Treating patients with an ionizing radiation dose that is within 5% of the prescribed dose is vital to their recovery, elimination of cancerous growth and is also required by the International Commission of Radiation Units and Measurements [ICRU Report 24, 1976]. Accordingly, performing quality assurance on the delivered treatment is important for exposing any inconsistencies in treatment delivery so that the prescribed dose for the fraction of delivered radiation in the next treatment can be altered to increase the efficacy of the treatment. Furthermore, ionizing radiation is employed in diagnostic tests and in interventional imaging procedures. The measurement of applied dose is of great value in these contexts.
Currently, there are a few techniques used to estimate the actual amount of radiation that is given to a patient. However, many of these techniques are not used during each treatment session, do not provide a real time estimate of the delivered radiation dose, or cannot be used in situ (i.e. placed within the tissue of the patient).
One way to estimate the dose to be delivered to a patient is to rely on a dose measured with an accepted dosimeter (MOSFET or ion chamber) within a phantom. A phantom is an inanimate representation of the patient for purposes of dosimetry. However, this technique does not provide the actual applied dose, and a conversion from dose in phantom to dose in patient would still have to be performed. It can give a general estimate of dose delivered to the volume treated and provide a rough shape of dose distribution. However, the best estimate of dose is always one that is measured during the actual treatment, as there are many differences between a phantom and an actual patient (i.e. properties of tissue vary within a patient and between patients, breathing motion, internal organ motion between treatments, etc).
One technique for measuring dose to the patient in current, limited use is to rely on thermal luminescence dosimeters (TLD), which are often used in the personal monitors that are worn by workers in environments in which radiation is used to determine the radiation exposure of the workers. The personal monitors contain radiation sensitive material that may be analyzed to determine radiation dose. Current accepted processing methods typically require post-exposure, high-temperature curing followed by rigorous processing steps, and therefore cannot provide a radiation dose measurement right away. The TLD material may be placed on the patient during radiation therapy and used to do initial measurements at the beginning of a patient's treatment program. However, because of the rigorous processing procedure required, at best, the dose is read out the same day as the treatment is delivered and not immediately after or during the treatment.
Another radiation dosimeter that is currently in limited use is MOSFET-based. The MOSFET-based radiation dosimeter includes metallic components and the signal is transmitted via metallic conductive wires. While it is possible to use metallic components in the dosimeter, the metallic components and wires interact with the ionizing radiation which may prevent accurate measurements from being made.
Another type of dosimeter is an ionization micro-chamber dosimeter that relies on electronics-based measurements, and has inner metallic components. The inside of the ionization micro-chamber, which can be as small as 2.0 mm3, is filled with air. The interaction of a volume of air with ionizing radiation is quite different than that of the same volume of water. Hence the air also contributes to the alteration of prescribed dose in the vicinity of the detector. Aside from these issues, ionization micro-chamber dosimeters are also quite expensive (i.e. approximately $5,000 CDN). At this price, it would be extremely difficult to implement such a dosimeter for daily patient dosimetry, unless each dosimeter was sterilized and reused (but this can also be costly). The ionization micro-chamber dosimeters are also complicated to use, and are generally operated by physicists.
Metallic wires may still be used in a dosimeter. However, it is beneficial to use another material that won't affect radiation delivery or radiation measurement, is cost-effective, and is safe and robust for use with patients. Further, there is a need for radiation dosimeters that can determine the amount of radiation that is delivered to the surface as well as the sub-surface of the patient during treatment. It is also desirable to have a radiation dosimeter that enables estimation of the delivered radiation dose quickly (in real-time), so that the medical practitioner can stop treatment and adjust the prescribed radiation dose during current treatment session if necessary.